The present invention relates to an ion guide or ion trap, a mass spectrometer, a method of guiding or trapping ions and a method of mass spectrometry.
Various ion trapping techniques are known in the field of mass spectrometry. Commercially available 3D or Paul ion traps, for example, provide a powerful and relatively inexpensive tool for many different types of organic analysis. 3D or Paul ion traps generally have a cylindrical symmetry and comprise a central cylindrical ring electrode and two hyperbolic end cap electrodes. In operation an RF voltage is applied between the end cap electrodes and the central ring electrode of the form:V0-pk(t)=V0 cos(σt)where V0 is the zero to peak voltage of the applied RF voltage and σ is the frequency of oscillation of the applied RF voltage.
The physical spacing and shape of the electrodes is such that a quadratic potential is maintained in both the radial and axial directions. Under these conditions ion motion is governed by Mathieu's equation and the various criteria for stable ion trapping are well known to those skilled in the art. The motion of the ions consists of a relatively low frequency component secular motion and a relatively high frequency oscillation or micro-motion which is directly related to the frequency at which the drive voltage is modulated.
Ions may be mass selectively ejected from a 3D or Paul ion trap by: (a) mass selective instability wherein either the amplitude and/or the frequency of the applied RF voltage is altered, (b) by resonance ejection wherein a small supplementary RF voltage is applied to one or both of the end cap or ring electrodes which has the same frequency as the secular frequency of the ions of interest, (c) by application of a DC bias voltage maintained between the ring electrode and the end cap electrodes, or (d) by combinations of the above techniques.
Ions are usually introduced into most commercial 3D or Paul ion traps from an external ion source via a small hole in one of the end cap electrodes. Once within the ion trap, the ions may then be cooled by collisions with a buffer gas to near thermal energies. This has the effect of concentrating the ions towards the centre of the trapping volume of the ion trap. Ions having a specific mass to charge ratio may then be mass selectively ejected from the ion trap. Ejected ions exit the ion trap through a small hole in the end cap electrode opposed to the end cap electrode having an aperture for introducing ions into the ion trap. The ions ejected from the ion trap are then detected using an ion detector.
3D or Paul ion traps suffer from the disadvantage that they possess a relatively limited dynamic range due to the fact that they have a relatively low space charge capacity. Furthermore, extreme care must be taken to ensure that correct conditions are maintained during ion introduction in order to minimize ion losses. As will be understood by those skilled in the art, injecting ions into a 3D Paul ion trap can be particularly problematic.
More recently linear ion traps have been developed and commercialised. Such ion traps generally comprise a multipole rod set wherein ions are confined radially within the ion trap due to the application of a RF voltage to the rods. Ion motion and stability in the radial direction is governed by Mathieu's equation and is well known. Ions may be contained axially within the linear ion trap by the application of a DC or RF trapping potential to electrodes at either end of the multiple rod set. Ion ejection may be accomplished by either ejecting ions radially from the ion trap through a slot in one of the rods or axially by using a combination of radial excitation and inherent field distortions at the axial boundary of the rods.
Linear ion traps generally exhibit increased ion trapping capacities relative to 3D or Paul ion traps and therefore linear ion traps generally exhibit a substantially higher dynamic range. Linear ion traps have an important advantage in that ions may be axially introduced into the ion trap and in some cases axially ejected from the ion trap in a direction which is orthogonal to the radial RF oscillating trapping potential. This enables ions to be transferred more efficiently into and out of the ion trap thereby resulting in improved sensitivity. Linear ion traps are therefore increasingly being preferred to 3D or Paul ion traps due to their increased sensitivity and relatively large ion trapping capacity.
Optimum performance of a linear ion trap which uses radial ejection rather than axial ejection may be achieved using a pure quadrupolar radial potential distribution and accurately shaped hyperbolic rods. However, deviations in the linearity of the radial confining field caused, for example, by mechanical misalignment of the rods can seriously compromise the performance of such a linear ion trap. The provision of slots in the rods of the linear ion trap to facilitate radial ejection can also lead to significant distortions in the radial field. These distortions can further degrade the performance of the linear ion trap. In addition during radial ejection it may be necessary to use more than one ion detector for efficient detection of the ejected ions. This adds to the overall complexity and expense of the ion trap.
It is known to eject ions axially from a linear ion trap. However, the performance of axial ejection of ions from a linear ion trap using fringe fields may also be affected by distortions in the linearity of the radial field. Axial ejection of ions relies upon efficient radial resonance excitation of the ions. If the radial field is non-linear then the resonant frequency will not be constant as the radius of the ion motion increases. Accordingly, the performance of the ion trap in this mode of operation will be compromised. A further problem with axially ejecting ions from a known linear ion trap is that only those ions at or close to the exit fringe field will actually be ejected from the ion trap. Accordingly, the theoretical gains in dynamic range and sensitivity of a linear ion trap relative to a 3D or Paul ion trap may be reduced in practice due to the relatively small region from which ions may actually be ejected from.
U.S. Pat. No. 5,783,824 (Hitachi) discloses a linear ion trap wherein an axial DC or electrostatic field is maintained along the length of the ion trap. Ions are ejected axially by resonance excitation by the application of a supplementary axial RF potential which oscillates at the fundamental harmonic frequency of the ions which are desired to be ejected. This known linear ion trap has the general advantages of other forms of linear ion trap but in addition forces ions to oscillate axially with a frequency characteristic of their mass to charge ratio. This facilitates axial resonance ejection of ions from the ion trap.
The linear ion trap disclosed in U.S. Pat. No. 5,783,824 uses resonance excitation to axially eject ions at the fundamental frequency of simple harmonic oscillation determined by an axial quadratic DC or electrostatic potential. However, in practice, it is difficult to generate a true axial quadratic potential due in part to field relaxation effects at the ends or boundaries of the ion trap. Deviations from a true quadratic axial DC or electrostatic potential will result in the frequency of oscillation of the ions being dependent upon the amplitude of oscillation of the ions and this will compromise the performance of the ion trap using resonance ejection.
It is therefore derived to provide an improved ion trap or ion guide.